The present invention relates generally to the field of radio-frequency and microwave microcircuit modules, and more particularly to liquid metal micro-switches used in such modules.
Microwaves are electromagnetic energy waves with very short wavelengths, typically ranging from a millimeter to 30 centimeters peak to peak. In high-speed communications systems, microwaves are used as carrier signals for sending information from point A to point B. Information carried by microwaves is transmitted, received, and processed by microwave circuits.
Packaging of radio frequency (RF) and microwave microcircuits has traditionally been very expensive and has required very high electrical isolation and excellent signal integrity through gigahertz frequencies. Additionally, integrated circuit (IC) power densities can be very high. Microwave circuits require high frequency electrical isolation between circuit components and between the circuit itself and other electronic circuits. Traditionally, this need for isolation has resulted in building the circuit on a substrate, placing the circuit inside a metal cavity, and then covering the metal cavity with a metal plate. The metal cavity itself is typically formed by machining metal plates and then attaching multiple plates together with solder or an epoxy. The plates can also be cast, which is a cheaper alternative to machined plates. However, accuracy is sacrificed with casting.
One problem attendant with the more traditional method of constructing microwave circuits is that the method of sealing the metal cover to the cavity uses conductive epoxy. While the epoxy provides a good seal, it comes with the cost of a greater electrical resistance, which increases the loss in resonant cavities and increases leakage in shielded cavities. Another problem with the traditional method is the fact that significant assembly time is required, thereby increasing manufacturing costs.
Another traditional approach to packaging RF/microwave microcircuits has been to attach gallium arsenide (GaAs) or bipolar integrated circuits and passive components to thin film circuits. These circuits are then packaged in the metal cavities discussed above. Direct current feed-through connectors and RF connectors are then used to connect the module to the outside world.
Still another method for fabricating an improved RF microwave circuit is to employ a single-layer thick film technology substrate in place of the thin film circuits. While some costs are slightly reduced, the overall costs remain high due to the metallic enclosure and its connectors, and the dielectric materials typically employed (e.g., pastes or tapes) in this type of configuration are electrically lossy, especially at gigahertz frequencies. The dielectric constant is poorly controlled as a function of frequency. In addition, controlling the thickness of the dielectric material often proves difficult.
A more recent method for constructing completely shielded microwave modules using only thick film processes without metal enclosures is disclosed by Lewis R. Dove, et al. in U.S. Pat. No. 6,255,730 entitled xe2x80x9cIntegrated Low Cost Thick Film RF Modulexe2x80x9d, hereinafter Dove. Dove discloses an integrated low cost thick film RF module and method for making same. An improved thick film dielectric is employed to fabricate three-dimensional, high frequency structures. The dielectrics used (KQ-120 and KQ-CL907406) are available from Heraeus Cermalloy, 24 Union Hill Road, West Conshohocken, Pa. These dielectrics can be utilized to create RF and microwave modules that integrate the I/O and electrical isolation functions of traditional microcircuits without the use of previous more expensive components.
Electronic circuits of all construction types typically have need of switches and relays. The typical compact, mechanical contact type relay is a lead relay. A lead relay comprises a lead switch, in which two leads composed of a magnetic alloy are contained, along with an inert gas, inside a miniature glass vessel. A coil for an electromagnetic drive is wound around the lead switch, and the two leads are installed within the glass vessel as either contacting or non-contacting.
Lead relays include dry lead relays and wet lead relays. Usually with a dry lead relay, the ends (contacts) of the leads are composed of silver, tungsten, rhodium, or an alloy containing any of these, and the surfaces of the contacts are plated with rhodium, gold, or the like. The contact resistance is high at the contacts of a dry lead relay, and there is also considerable wear at the contacts. Since reliability is diminished if the contact resistance is high at the contacts or if there is considerable wear at the contacts, there have been various attempts to treat the surface of these contacts.
Reliability of the contacts may be enhanced by the use of mercury with a wet lead relay. Specifically, by covering the contact surfaces of the leads with mercury, the contact resistance at the contacts is decreased and the wear of the contacts is reduced, which results in improved reliability. In addition, because the switching action of the leads is accompanied by mechanical fatigue due to flexing, the leads may begin to malfunction after some years of use.
A newer type of switching mechanism is structured such that a plurality of electrodes are exposed at specific locations along the inner walls of a slender sealed channel that is electrically insulating. This channel is filled with a small volume of an electrically conductive liquid to form a short liquid column. When two electrodes are to be electrically closed, the liquid column is moved to a location where it is simultaneously in contact with both electrodes. When the two electrodes are to be opened, the liquid column is moved to a location where it is not in contact with both electrodes at the same time.
To move the liquid column, Japanese Laid-Open Patent Application SHO 47-21645 discloses creating a pressure differential across the liquid column is created. The pressure differential is created by varying the volume of a gas compartment located on either side of the liquid column, such as with a diaphragm.
In another development, Japanese Patent Publication SHO 36-18575 and Japanese Laid-Open Patent Application HEI 9-161640 disclose creating a pressure differential across the liquid column by providing the gas compartment with a heater. The heater heats the gas in the gas compartment located on one side of the liquid column. The technology disclosed in Japanese Laid-Open Patent Application 9-161640 (relating to a microrelay element) can also be applied to an integrated circuit. Other aspects are discussed by J. Simon, et al. in the article xe2x80x9cA Liquid-Filled Microrelay with a Moving Mercury Dropxe2x80x9d published in the Journal of Microelectromechanical Systems, Vol. 6, No. 3, Sep. 1997. Disclosures are also made by You Kondoh et al. in U.S. Pat. No. 6,323,447 entitled xe2x80x9cElectrical Contact Breaker Switch, Integrated Electrical Contact Breaker Switch, and Electrical Contact Switching Methodxe2x80x9d.
There remains a need for an electrically isolated liquid metal micro-switch for use in an integrally shielded high-frequency microcircuit.
The present patent document relates to techniques for fabricating electrically isolated liquid metal micro-switches in integrally shielded microcircuits. Disclosures made herein provide means by which liquid metal micro-switches can be integrated directly into the construction of shielded thick film microwave modules.
In a representative embodiment, a liquid metal micro-switch comprises a first substrate and a first ground plane which is attached to the first substrate. A first dielectric layer is attached to the first ground plane. A conductive signal layer is attached to the first dielectric layer and patterned so as to define first, second, and third signal conductors having respectively first, second, and third micro-switch contacts. A second dielectric layer is attached to the signal layer conductors and to the first dielectric layer. a second ground plane is attached to the second dielectric layer. A second substrate is attached to the second dielectric layer and has a cavity. A third ground plane is attached to the second substrate. A heater is positioned inside the cavity. A main channel is partially filled with a liquid metal, wherein the main channel encompasses the micro-switch contacts. A sub-channel connects the cavity and main channel, wherein a gas fills the cavity and sub-channel and wherein heater activation forces an open circuit between first and second micro-switch contacts and a short circuit between second and third micro-switch contacts.
In another representative embodiment, a liquid metal micro-switch comprises a first substrate and a first ground plane, wherein the first ground plane is attached to the first substrate. A first dielectric layer is attached to the first ground plane. A conductive signal layer is attached to the first dielectric layer and patterned so as to define first, second, and third signal conductors, wherein the first, second, and third signal conductors have respectively first, second, and third micro-switch contacts. A second ground plane is attached to a second substrate. A second dielectric layer is attached to the second substrate, has a cavity, and is attached to the first dielectric layer. A heater is positioned inside the cavity. A main channel is partially filled with a liquid metal with the main channel encompassing the micro-switch contacts. A sub-channel connects the cavity and main channel with a gas filling the cavity and sub-channel, wherein heater activation forces an open circuit between first and second micro-switch contacts and a short circuit between second and third micro-switch contacts.
In still another representative embodiment, a method for fabricating a liquid metal micro-switch comprises attaching a first ground plane to a first substrate, attaching a first dielectric layer to the first ground plane, and attaching a conductive signal layer to the first dielectric layer. The conductive signal layer is patterned so as to define first, second, and third signal conductors which have respectively first, second, and third micro-switch contacts. A second dielectric layer is attached to first, second, and third signal conductors and to the first dielectric layer. The second dielectric layer is patterned so as to define at least one sub-channel and a main channel. A second ground plane is attached to the second dielectric layer. A cavity is created in a second substrate. A third ground plane is attached to the second substrate. A heater is attached inside the cavity. The main channel is partially filled with a liquid metal, wherein the main channel encompasses the micro-switch contacts. The second substrate and the third ground plane are attached to the second ground plane and the second dielectric layer.
In yet another representative embodiment, a method for fabricating a liquid metal micro-switch comprises attaching a first ground plane to a first substrate, attaching a first dielectric layer to the first ground plane, and attaching a conductive signal layer to the first dielectric layer. The conductive signal layer is patterned so as to define first, second, and third signal conductors having respectively first, second, and third micro-switch contacts. A second ground plane is attached to a second substrate. A second dielectric layer is attached to the second substrate. The second dielectric layer is patterned so as to define a cavity, at least one sub-channel, and a main channel. A second dielectric layer is attached to first, second, and third signal conductors and to the first dielectric layer. A heater is attached inside the cavity. The main channel is partially filled with a liquid metal, wherein the main channel encompasses the micro-switch contacts. The second dielectric layer is attached to the conductive signal layer and to the first dielectric layer.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.